An internal combustion (IC) engine may include an exhaust gas recirculation (EGR) system for controlling the generation of undesirable pollutant gases and particulate matter. EGR systems recirculate exhaust gases and particular matter into the intake air supply of the IC engine. The exhaust gases, which are recirculated to the engine cylinders, reduce the concentration of oxygen in the cylinders, which lowers the maximum combustion temperature in the cylinders and slows the chemical reaction of the combustion process. As a result, a decrease in nitrous oxides (NOx) formation is achieved. Furthermore, the exhaust gases typically contain unburned hydrocarbons which are burned upon recirculation the engine cylinder and which further reduces the emission of exhaust gas by-products.
An IC engine may also include one or more turbochargers for compressing air which is supplied to one or more combustion chambers of corresponding combustion cylinders. Each turbocharger typically includes a turbine driven by exhaust gases of the engine and a compressor which is driven by the turbine. The compressor receives the air to be compressed and supplies the compressed air to the combustion chambers. The compressor may also be used to compress a fuel/air mixture as well as air.
When utilizing EGR in a turbocharged diesel engine, the exhaust gases to be recirculated may be removed upstream of the exhaust gas driven turbine associated with the turbocharger. In many EGR applications, the exhaust gas is diverted by a poppet-type EGR valve downstream from the exhaust manifold. The percentage of the total exhaust flow which is diverted for introduction into the intake manifold of the engine is known as the “EGR rate” of the engine. One example of an EGR system can be found in U.S. Pat. No. 6,128,902, which discloses an EGR valve 34 disposed in a conduit 32 that connects the exhaust manifold 28 to the intake manifold 26.
Variable-geometry turbochargers (VGTs) are a family of turbochargers, usually designed to allow the effective aspect ratio (sometimes called A/R ratio) of the VGT turbine to be altered as conditions change. A VGT turbine typically has a set of movable vanes to control pressure of the exhaust flowing through the VGT turbine. At low engine speeds when exhaust flow is low, the vanes are partially closed to accelerate the VGT turbine. Accelerating the VGT turbine increases boost pressure delivered by the compressor that is driven by the VGT turbine. As the engine speed increases, the vanes are opened to slow down the VGT turbine. Slowing down the VGT turbine prevents the boost pressure provided by the compressor from reaching excessive levels.
VGTs have proven useful because an optimum aspect ratio at low engine speeds is different from an optimum aspect ratio at high engine speeds. If the aspect ratio is too large, the turbocharger may fail to create boost at low speeds; if the aspect ratio is too small, the turbocharger may choke the engine at high speeds, leading to high exhaust manifold pressures, high pumping losses, and ultimately lower power output.
By altering the vane positions as the engine accelerates, the aspect ratio of the VGT turbine can be maintained at its optimum. As a result, VGTs have a minimal amount of lag, have a low boost threshold, and are very efficient at higher engine speeds. VGTs tend to be much more common on diesel engines because the lower exhaust temperatures of diesel engines means the VGTs are less prone to failure.
Selective catalytic reduction (SCR) systems catalytically convert NOx to nitrogen and water. A gaseous reductant, typically urea or ammonia, is added to the exhaust gas stream where it is adsorbed onto the catalyst. Carbon dioxide is a reaction product when urea is used as the reductant. Because of the need to supply both a reductant and a catalyst, SCR systems tend to be space intensive and are most appropriate for large utility boilers, industrial boilers, and municipal solid waste boilers. However, due to increasingly stringent emission standards, recent applications include diesel engines. Further, because current EGR systems do not meet Tier4 admission standards, the combination of EGR and SCR systems in diesel engine designs has become common.
However, an SCR system requires the driver or maintenance staffer to replenish an on-board urea or ammonia tank that contains the reactant in an aqueous solution. Further, operators must buy and store the solution or have drivers find it while on the road. A pump pushes the solution out of the tank. Because the solution is about two-thirds purified water, a heater is used in the tank or line between it and the dosing chamber, where solution is injected downstream of the particulate filter. This equipment plus the solution, which weighs about 9 pounds per gallon, adds 200 to 400 pounds and occupies precious space on a truck. This can be a major drawback to any weight-conscious owner and presents packaging problems for manufacturers of diesel trucks and other equipment that includes an SCR.
Thus, there is a need for an improved emission control system for internal combustion engines that can meet the new stringent emission requirements in terms of NOx, yet avoid the disadvantages of SCR systems and the combination of EGR and SCR systems.